A system to which electrophotographic technology is adopted has conventionally been used in a copying machine, a printer, a multifunction printer, a production printer and the like. In recent years, it is sometimes required to form various value-added images, for example, a see-through design image (watermark) using a clear toner.
By forming a plurality of image parts each having a different gloss level, a watermark can be recognized based on a difference in gloss level, not in color tone, among the plurality of image parts.
Also, in an image forming method in which a color toner is used, an image containing a photograph image part and a letter image part, for example, is formed by increasing the gloss level of the photograph image part and relatively decreasing the gloss level of the descriptive text (the letter image part). Accordingly, the photograph image part is vividly seen, while the descriptive text is easy to read. In this manner, it is sometimes required to form a plurality of image parts each having a different gloss level on the same image.
As a formation process of an image having varied gloss levels, there are known a process of, in case of using two or more types of toners, varying a laminating order or a toner attachment amount of toner images (see Patent Literature 1), a process of heat-fixing toners each having a varied melt viscosity (see Patent Literature 2) and the like.
However, in the above Patent Literature 1, the gloss level can be varied within some range from a certain predetermined gloss level, but the range of a gloss level that can be varied is up to 5 or so at 20° gloss. This range is not necessarily sufficient to form a gloss level difference among the image parts on the same image for exerting design features. Also, the above Patent Literature 2 discloses an image forming method of using two types of clear toners, which are a clear toner capable of forming a high gloss level image and a clear toner capable of forming a low gloss level image. In this case, one of the clear toners is selected in an alternative manner in order to simply form either the high gloss level image or the low gloss level image. Thus, there has been a problem that a plurality of image parts each having a different gloss level cannot be formed on the same image.
Also, there has been a problem that process conditions associated with image formation become complicated in both processes when attempting to realize the image forming method described above using the method disclosed in Patent Literature 1 or 2.